As a typical example in which the performance as stated above is required, there is a casing of an electric motor for a power window disposed in the inside of a door of a vehicle. That is, even when a vehicle falls in water because of an accident or the like, in order to enable a passenger to escape, it is desired that even if the electric motor is submerged, the electric motor can be operated by a switch operation from the inside of the vehicle for a certain time (for example, five minutes) after the submergence, so that the window can be opened.
For this purpose, when the casing is completely sealed by a sealant or the like, the object of preventing the infiltration of water can be achieved. However, in normal use, heat generation from the operation of the motor causes the air inside of the casing to be heated and expanded, and there occurs such a problem that a sealing portion of the casing is broken by the air expansion. Once the sealing portion is broken, the passage of water at this portion can occur, and it becomes difficult to achieve the object of preventing the infiltration of water.
In order to solve the problem, it is proposed that a vent hole is formed through a part of a casing in an inward-outward direction and this vent hole is closed with a permeable sheet.
This permeable sheet has such minute permeable pores (their size being 0.05 to 20 μm) as to allow gas (oxygen gas, nitrogen gas, etc.) constituting the air, and moisture or water vapor (its size being 0.0004 μm) in the air to pass through, and to prevent the passing of even the smallest water droplet (water droplet of a drizzle state having a size of 100 μm). The permeable sheet is bonded to the vent hole portion of the casing by a doughnut-shaped adhesive agent or tackiness agent.
However, at the bonding portion, the performance of the adhesive agent or the tackiness agent deteriorates in long-term use, the bonding portion peels off, and water is able to infiltrate through the peeled portion. Further, there still remains a problem that the consistency of quality is lacking.
Besides, in the bonding work of the permeable sheet, since an adhesive agent or a tackiness agent is used and the bonding is carried out in a pressing state, the pressing force applied to this portion is liable to be partially varied, and this variation also becomes a factor negatively effecting the bonding quality. Further, since the permeable sheet is thin and small, the adhesion work is complicated, and excessive attention is needed for the adhesion work in order to avoid damage to the permeable sheet.